Ising model with isotropic competing interactions in the presence of a field

Summary We consider a spin system with competing interactions isotropic with respect to the axes of a cubic lattice in the presence of an external field. We show that for small values of the external fieldH, the paramagnetic to modulated phase transition is fluctuation-induced first order, while for larger fields, such transition changes to continuous at a tricritical point. Applications for fluids systems are proposed. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

Crystal engineering of hydroxy group hydrogen bonding: design and synthesis of new diol lattice inclusion hosts

The diols 7-11 have been synthesized, and their X-ray crystal structures determined, to learn how to influence and control lattice hydroxy group hydrogen bonding using crystal engineering ideas. To obtain new lattice inclusion hosts precise structural rules can be defined which enable the necessary supramolecular interactions to be duplicated. In this manner the helical tubuland 10 and ellipsoidal clathrate 11 hosts were obtained for the first time and their chloroform inclusion compounds characterized. New synthetic routes were utilized to obtain the bicyclo[3.3.2]decane and 9-thiatricyclo[4.3.1.1(3,8)]undecane frameworks present in these compounds. The solid-state conformations of bicyclo[3.3.2]decane derivatives 9 and 10 are compared with prior predictions and studies made on this uncommon ring system.     

Probing the interaction between fluorophores and DNA nucleotides by fluorescence correlation spectroscopy and fluorescence quenching

We have investigated the association interactions between the fluorescent dyes TAMRA, Cy3B and Alexa-546 and the DNA deoxynucleoside monophosphates by means of fluorescence quenching and fluorescence correlation spectroscopy (FCS). The interactions of Cy3B and TAMRA with the nucleotides produce a decrease in the apparent diffusion coefficient of the dyes, which result in a shift toward longer times in the FCS autocorrelation decays. Our results with Cy3B demonstrate the existence of Cy3B-nucleotide interactions that do not affect the fluorescence intensity or lifetime of the dye significantly. The same is true for TAMRA in the presence of dAMP, dCMP and dTMP. In contrast, the diffusion coefficient of Alexa 546 remains practically unchanged even at high concentrations of nucleotide. These results demonstrate that interactions between this dye and the four dNMPs are not significant. The presence of the negatively charged sulfonates and the bulky chlorine atoms in the phenyl group of Alexa 546 possibly prevent strong interactions that are otherwise possible for TAMRA. The characterization of dye-DNA interactions is important in biophysical research because they play an important role in the interpretation of energy transfer experiments, and because they can potentially affect the structure and dynamics of the DNA.     

Pyridine Syntheses. II. Condensation Routes Toward Streptonigrin Ring C

Alternative complimentary syntheses of penta-substituted pyridine rings with full regiochemical control of substituents were studied as a method for the synthesis of Streptonigrin ( 1 ). Various α-substituted acetophenones 2 were reacted with enones 3 in acetic acid/ammonium acetate and air to afford penta-substituted pyridines 4 . α-Substituents that could provide a source of exocyclic nitrogen at position 3 of these Steptonigrin ring-C models proved to be the limiting factor. However, an inverse “3+2+1” cyclocondensation of α-cyanochalcone 5c with 2-furyl ethyl ketone ( 6b ) afforded the desired model 6-(2-furyl)-5-methyl2,4-diphenyl-3-pyridinecarbonitrile ( 4g ) in 75% yield.     

Core-softened fluids, water-like anomalies, and the liquid-liquid critical points

Molecular dynamics simulations are used to examine the relationship between water-like anomalies and the liquid-liquid critical point in a family of model fluids with multi-Gaussian, core-softened pair interactions. The core-softened pair interactions have two length scales, such that the longer length scale associated with a shallow, attractive well is kept constant while the shorter length scale associated with the repulsive shoulder is varied from an inflection point to a minimum of progressively increasing depth. The maximum depth of the shoulder well is chosen so that the resulting potential reproduces the oxygen-oxygen radial distribution function of the ST4 model of water. As the shoulder well depth increases, the pressure required to form the high density liquid decreases and the temperature up to which the high-density liquid is stable increases, resulting in the shift of the liquid-liquid critical point to much lower pressures and higher temperatures. To understand the entropic effects associated with the changes in the interaction potential, the pair correlation entropy is computed to show that the excess entropy anomaly diminishes when the shoulder well depth increases. Excess entropy scaling of diffusivity in this class of fluids is demonstrated, showing that decreasing strength of the excess entropy anomaly with increasing shoulder depth results in the progressive loss of water-like thermodynamic, structural and transport anomalies. Instantaneous normal mode analysis was used to index the overall curvature distribution of the fluid and the fraction of imaginary frequency modes was shown to correlate well with the anomalous behavior of the diffusivity and the pair correlation entropy. The results suggest in the case of core-softened potentials, in addition to the presence of two length scales, energetic, and entropic effects associated with local minima and curvatures of the pair interaction play an important role in determining the presence of water-like anomalies and the liquid-liquid phase transition.     

(Nb2W4O19), TMA2, Na4(OH2)14(SO4): a new layered structure with Lindqvist heteropolyanions, XAS characterization of the HPAs

The new (Nb₂W₄O₁₉),TMA₂, Na₄(OH₂)₁₄(SO₄) has been evidenced as a minor phase during the Nb₂W₄O₁₉TMA (tetramethylammonium) salt synthesis. Its crystal structure has been refined from single crystal X-ray diffraction data, system monoclinic, a=10.166(5) Å, b=17.93(1) Å, c=24.81(1) Å, β=93.057(7)°, space group (S.G.) C2/c, Z=4, R₁=3.96%, wR₁=4.50%. It shows the stacking of cationic and anionic bidimensional layers. The anionic layer of formula [(Nb₂W₄O₁₉), TMA₂ ]²⁻ is formed of isolated Lindqvist HPAs surrounded by TMA groups. The isolated layers adopt a trigonal symmetry that is lost in the crystal by the association of the cationic sheets. These later, of formula [Na₄(OH₂)₁₄(SO₄)]²⁺ form porous net-like sheets with nearly circular cavities of diameter 7.5 Å. groups host the available cavities in a disordered manner. The cohesion between the sheets is performed by both electrostatic interactions and a set of hydrogen bonds. In the cationic layers, the highly symmetrical surrounding of HPAs by TMA groups yields a homogeneous electrostatic field at their external surface leading to a statistic Nb/W disorder over the three available independent metallic positions. Then, XAS experiments at the L₁/L₃-W edge complementarily helped to highlight the preferential cis configuration of (Nb₂W₄O₁₉)⁴⁻ anions, help to the strong Nb vs W contrast in their contribution to the backscattering paths. Previously to these experiments, it was of course checked that both the two phases present in the prepared sample contain Nb₂W₄O₁₉ anions with nearly unchanged geometry.     

Test study on the excitation spectra of the N<Subscript>2</Subscript>–He van der Waals molecule

We have performed ab initio fourth-order Møller–Plesset perturbation theory calculations in the framework of the supermolecule approach on the vertical excitation spectra of the weakly bound van der Waals N₂–He dimer. They indicate a ``T-shaped' stablest ground N<sub>2</sub>(X<sup>1</sup><sub>g</sub><sup>+</sup>)–He(<sup>1</sup>S) electronic state with a well depth, D<sub>e</sub>, of 21.63 cm<sup>–1</sup> at a minimum distance, R<sub>e</sub>, of 3.44 Å and zero-point vibration correction, D<sub>o</sub>, of 7.07 cm<sup>–1</sup>. They also indicate a ``T-shaped' stablest excited conformer with R_e=3.25 Å, D_e=36.85 cm^–1 and D_o=17.06 cm^–1 for the N₂(B³_g)–He(¹S) triplet electronic level. In order to investigate the use of less-demanding correlation methods, test density functional theory calculations using the mPW1PW exchange–correlation functional are also presented for comparison.     

SERRS study of [Ru(CN)<Subscript>5</Subscript>(pyS)]<Superscript>4−</Superscript> SAM and cytochrome c: A suggestion toward the heterogeneous molecular recognition

Surface-Enhanced Raman Scattering (SERS) spectra of [Ru(CN)₅(pyS)]⁴⁻ (RupyS) complex self-assembled monolayer (SAM) were obtained on gold and silver surfaces at 632.8 and 413.1 nm excitation radiations, respectively. The bands assigned to the heme iron of the cytochrome c (cyt c) metalloprotein group were observed by using the RupyS SAM on silver at 413.1 nm. The Surface-Enhanced Resonance Raman Scattering (SERRS) spectra of the RupyS SAM on silver in the cyt c solution obtained at −0.2 and +0.2 V present bands at 1,365 and 1,374 cm⁻¹ characteristic of the heme group, indicating the reduced and oxidized states of this protein, respectively. The bands observed at 1,464, 1,504, and 1,638 cm⁻¹ are used to confirm the redox state of cyt c. The presence of the oxidized and reduced bands in function of different applied potential is an evidence that the protein is interacting with the modifier. This paper is dedicated to Prof. Francisco Nart, in memoriam.     

Complexation of Ethanol by a Helical Tubuland Diol Host

The cage compound 4,7-dimethylpentacyclo[6.3.0.0^2,6.0^3,10.0^5,9]undecane-anti-4,anti-7-diol 3 and its 4,7-bis(d ₃-methyl) derivative 4 form 1:1 cocrystalline complexes with ethanol instead of the expected helical tubulate inclusion compounds. The X-ray crystal structure of (4)·(ethanol) [(C₁₃H₁₂D₆O₂)·(C₂H₆O), Cmca, a 19.049(2), b 14.932(2), c 10.117(1) Å, Z 8, R 0.061] shows that the key supramolecular synthon is a hydrogen-bonded (O — H)₆ cycle of hydroxy groups constructed from two ethanols (in ring positions 1 and 4) and four diols. This uncommon centrosymmetric motif permits efficient lattice packing of ethanol and both diol enantiomers into layers without requiring the self-resolution that would be mandatory in forming the helical tubulate structure.